Method of manufacturing grinding wheels and the like



A ril 9, 1968 v. K. CHARVAT 3,377,411

METHOD OF MANUFACTURING GRINDING WHEELS AND THE LIKE Filed March 2, 1964 5 Sheets-Sheet l INVENTOR. VkRA/ON K CHARVAT APYil 1968 v. K. CHARVAT 3,377,411

METHOD OF MANUFACTURING GRINDING WHEELS AND vTHE LIKE Filed March 1964 5 Sheets-Sheet 2 9 F I G \ooeom FIG. I l -TOOL- -TOOI.

f VERNON 2222 1247" 7 BY W am,me mwwu A1mri1l9,1968 v. K. CHARVAT 3,377,411

METHOD OF MANUFACTURING GRINDING WHEELS AND THE LIKE Filed March 2, 1964 5 Sheets-Sheet 3 INVENTOR. VERNON ff [mm Ar United States Patent 3,377,411 METHOD OF MANUFACTURING GRINDING WHEELS AND THE LIKE Vernon K. Charvat, Bay Village, Ohio, assiguor to The Osborn Manufacturing Company, Cleveland, Ohio, a corporation of Ohio Continuation-impart of application Ser. No. 304,002, Aug. 23, 1963. This application Mar. 2, 1964, Ser. No. 348,735

29 Claims. (Cl. 26445) ABSTRACT OF THE DISCLOSURE A method of manufacturing grinding wheels and the like wherein granular abrasive is concentrated in substantially contacting relationship in a liquid resin binder by centrifuging the abrasive-binder mixture, and such abrasive grains are then very slightly uniformly spaced apart by controlled foaming of such binder prior to final setting of the binder.

Disclosure This invention relates as indicated to a method of manufacturing abrading tools such as grinding wheels and the like in a manner to achieve substantial efficiencies in production as well as a much improved abrading tool. This application is a continuation-in-part of my co-pending application Ser. No. 304,002, file-d Aug. 23, 1963, entitled, Abrading Tools; which is inturn a continuation-in-part of application Ser. No. 829,665, filed July 27, 1959, and now abandoned, entitled, Abrading Tools. This application is also a continuation-in-p-art of my coapending application Ser. No. 813,377, filed May 15, 1959, entitled, Abrading Tools; Ser. No. 854,468, filed Nov. 20, 1959 now Patent No. 2,996,075, entitled, Method of Making Articles From Foamed Elastomeric Material; Ser. No. 12,303, filed Mar. 2, 1960 now Patent No. 3,117,794, entitled, Composit-ion and Method for Making Grinding Wheels and the Like; and Ser. No. 15,135, filed Mar. 15, 1960 and now abandoned, entitled, Method of and Apparatus for Centrifugal Molding.

A grinding wheel, in contrast to a polishing or finishing wheel, is capable of making a cut of substantial depth in a work-piece which may be of cast iron or steel, for example, and the characteristics of prior grinding wheels are well known and described, for example, in The Grinding Wheel by Kenneth B. Lewis, published 1959, by The Grinding Wheel Institute, Cleveland, Ohio. Ordinarily, such grinding wheels have comprised a mass of densely compacted discrete abrasive grains bonded together by a molded and fired ceramic material or a resin bonding agent. Such Wheels have been notoriously difficult of manufacture, requiring very careful placement of the granular abrasive in a mold and usually rather lengthy baking or curing periods. Many of them are quite fragile or brittle and easily fracture if carelessly handled. They also require frequent dressing to ensure maintenance of the desired tool face profile to obtain a uniform cu t. Those wheels which have been hard enough to be capable of a fast or deep cutting action such as is needed for abrasive machining, requiring imposition of high unit pressures, have not also been capable of simultaneously producing a surface finish of the quality desired and frequently cause metallurgical damage to the Work. Consequently, in very many cases a preliminary rough grinding step has required a subsequent finishing operation and indeed a conventional cutting tool such as a milling cutter may first be employed followed by a rough grinding step and then a finishing step.

Polishing tools such as polishing pads and wheels have also been known in which polishing materials have been 3,377,411 Patented Apr. 9, 1968 ice incorporated in a body of readily yielding elastomeric material such as natural and artificial rubber and various synthetic resins. While suitable for use in cleaning or polishing operations, such articles have lacked entirely the dimensional stability and rigidity necessary in a grinding wheel which must remove stock accurately in amounts generally measured in thousandths of an inch and which must do so in a precise path or line of cut to achieve the desired dimension and geometry of the work.

In contrast to the tools generally described above, I have found that by the proper placement and concentration of abrasive grains or granules in a selected resin or plastic binder I have been able to produce an improved abrasive tool, and particularly a grinding wheel, wherein the granular abrasive material is disposed within the binder matrix or body in such manner as to achieve a tool which is essentially rigid considered as a whole, but wherein the individual abrasive grains exposed at the working face of the tool are slightly spaced apart and capable of individual micro-movement or adjustment relative to each other without being dislodged from their sockets in the binder, the binder being capable of a limited amount of local elastic deformation when exceptionally high pres.- sures are imposed on the individual exposed grains. In consequence, a grinding tool in accordance with this invention may be fed rapidly into the work to produce a relatively deep out without producing either excessive scoring of the work surface or prematurely dislodging excessively protruding grains in the tool face which would result in rapid break-down of the tool profile. Any such excessively protruding grains are forced inwardly of the tool face under the operating pressure imposed thereon until substantially all of the grains exposed in the work-ing face of the tool bear against and act upon the work; this 7 being achieved, however, without appreciable distortion of the tool considered as a whole, so that a dimensionally true cut is produced.

It is accordingly an important object of my invention to provide a novel abrading tool capable of fast cutting action but which nevertheless will neither itself break down prematurely or produce excessive scoring of the work, but instead will produce an exceptionally good finish for such a rapid out.

Another object is to provide an abrading tool capable of fast cutting action to produce a true dimensionally accurate grinding out without, however, also producing rapid erosion of the too-l edges (which would necessitate frequent dressing and machine down time) or causing metallurgical damage to the work.

Still another object is to provide an abrading tool, and especially a grinding tool, having abrasive grains uniformly slightly spaced apart in an essentially rigid non-brittle resin to afford a large number of cutting points exposed at the work-ing face of the tool which are slightly individually adjustable under working pressures imposed thereon due to stubborn elastic yielding action of the resin bond without affecting the essentially rigid character of the tool body.

A further object is to provide such tool having a multitude of very small closed cells in the resin bond thus spacing the individual grains slightly apart and incapable of absorbing water or other liquid to avoid imbalance of the tool in wet grinding, for example.

A still further object is to provide an abrading tool such as a grinding wheel which is not fragile and will not fracture easily when dropped or otherwise maltreated.

Another object is to provide a method of manufacturing such abrading tool, and especially a grinding wheel, which is rapid and inexpensive in that uniformly reproducible results are obtainable without appreciable production of rejects, utilizing centrifugal force preliminarily to distribute the granular abrasive material in a body of liquid resin, with the viscosity of the binder resin thereupon being increased and a foaming action produced properly to space the individual grains in the now viscous binder following cessation of effective centrifuging, such binder then being gelled or set to the desired rigid but non-brittle condition capable of local stubborn elastic yielding action when excessive pressure is imposed upon an individual abrasive granule exposed at the working face of the tool.

While a variety of resin or plastic bonding agents are suitable for employment in accordance with my invention and are commercially available, I particularly prefer certain selected polyurethane compositions which, when properly employed, evidence the desired physical characteristics mentioned above. Such polyurethane compositions are capable of foaming with the assistance of the small amount of moisture normally present and the various other resins may be caused to foam through the provision of appropriate well-know foaming agents activated by means of heat, catalysts and the like.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail certain illustrative embodiments of .the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In said annexed drawing:

FIG. 1 is a diagrammatic elevation partly in cross-section of a circular mold mounted upon a turntable or centrifuge and adapted for the production of a rotaary abrading wheel in accordance with the invention;

FIG. 2 is a view similar to FIG. 1 but illustrating a further stage in the process;

FIG. 3 is a vertical section illustrating a subsequent stage in the process;

FIG. 4 is a vertical section through the closed mold in the blowing and gelling or setting stage;

FIG. 5 is a view of a typical abrading wheel produced in accordance with this invention;

.FIG. -6 is a fragmentary transverse section on an enlarged scale taken on the line 6-6 on FIG. 5;

FIG. 7 is a magnified diagrammatic view indicating the disposition of the abrasive grains and binder resin during initial centrifuging;

FIG. 8 is a magnified view similar to FIG. 7 but indicating the relationship and form of such grains and binder subsequent to blowing and setting of the binder;

FIG. 9 is a magnified view on the same scale as FIG. 8 showing the cells formed in the radially inner non abrasive portion of the wheel;

FIG; 10 is a magnified diagrammatic view in crosssection showing only the abrasive grains protruding at the working face of the tool;

FIG. 11 is a magnified diagrammatic view indicating the manner in which the disposition of such grains is modified by the working pressure imposed thereon during performance of a grinding operation; and

FIGS. 12, 13 and 14 are semi-diagrammatic top plan views of a rotary mold showing a sequence of steps in the production of a grinding wheel or the like in accordance with this invention.

The preferred method of making the improved tools of the present invention will first be generally described, reference being had to the foregoing figures of the drawmg. In such description, the matrix or hinder material will generally be referred to as a resin, or more specifically as a polyurethane composition. It will be understood, however, that various resins or plastic compositions, and especially thermosetting plastic compositions may be utilized; for example: the reaction products of a member selected from the group consisting of an aromatic polyether and an aromatic polyester with a polyisocyanate, certain epoxy resin compositions, certain phenolic resin compositions, and certain silicone resins. In general, for

Method of manufacture Utilizing the preferred polyurethane resin composition as the binder material, the abrasive material may be incorporated in the unreacted or partially reacted polyurethane constituent mixture and the reaction then completed in an appropriate mold to form the desired abrading tool such as a grinding wheel. A blowing or foaming agent will ordinarily be incorporated in the mixture substantially simultaneously with the incorporation of the abrasive therein, and the blowing operation which occurs prior to final setting of the resin assists in accomplishing uniformly spaced distribution of the abrasive grains through the resin body and in maintaining such grains in suspension prior to solidification. Furthermore, the prompt setting of the polyurethane likewise militates against settling of the grains after these have thus become properly located.

In the production of many types of abrading tools, and especially grinding wheels for example, it is preferred to employ a method of the type diagrammatically illustrated in FIGS. 1-4 inclusive of the drawing. An annular mold 1 is shown having its base 2 inset in a turntable 3 adapted to be rotated about its vertical axis by worm gear unit 4 driven by electric motor 5. The mold is provided with a removable cover plate 6 having a central opening through which protrudes an axial stud 7 having a threaded reduced outer end portion 8. In the initial step of 'the operation, a measured quantity of the fluid or unset resin such as the mixture of polyurethane reactant constituents may be discharged from upper reservoir 9 into the central opening 10 of the mold cover plate 6 and the turntable 3 is revolved at a speed sufiicient to cause the resin to fiow radially outwardly and accumulate in the radially outer portion of the mold as shown at 11. In this manner, approximately the radially outer half of the mold may be filled.

Now referring to FIG. 2 which illustrates a subsequent operation atthe same station, a measured quantity of granular abrasive may next be discharged from hopper 12 into the rotating mold which will be rotated at a sufiicient- 1y high speed to cause such abrasive to fiow radially outwardly under the influence of centrifugal force into the previously deposited resin through which it migrates toward the radially outer periphery or circumferential portion of the mold cavity, accumulating in uniform manner in the radially outer circumferential region 13 in concentrated contacting or substantially contacting relationship with a relatively small quantity of the resin filling the interstices between the abrasive grains.

In the next stage of the operation, at the same station, additional resin may be discharged into the central region of the mold as shown in FIG. 3, but still ordinarily preferably not entirely filling the mold cavity The mold is thus now partially filled with the resin-abrasive mixture with the abrasive grains uniformly circumferentially distributed in a radially outer annular region of the mold and with the remainder of the resin body being substantially abrasive free. The resin constituents have been sufficiently liquid to permit such centrifugingbut as they continue to react, the viscosity rapidly increases. Furthermore, the binder now tends to foam, but such tendency may be largely inhibited during these preliminary stages of the manufacturing process due to the centrifugal force imposed on the rapidly rotating mass. When effective centrifuging is now ceased, ordinarily by completely stopping rotation of the turntable, a foaming of the resin body takes place in the now quite viscous resin, producing relatively small, ordinarily closed cells in the interstices between the abrasive grains to an extent sufficient to space the latter apart preferably not more than one grain diameter, with the entire resin-abrasive body accordingly expanding radially inwardly of the mold which is centrally vented as previously indicated. A multitude of small cells are simultaneously generated in the radially inner non-abrasive portion of the resin body, such cells ordinarily being smaller than those formed between the abrasive grains in the outer portion of the mold and having thinner walls or webs therebetween than the webs between the cells formed in such outer portion in which the abrasive grains are embedded.

An annular tapered plug 14 may be employed to close the mold as shown in FIG. 4, secured by an outer washer 15 and nut 16 threaded on stud 8. The resin binder within the mold is now permitted to set or cure, as may be required, to essentially rigid but non-brittle condition. In the case of the preferred polyurethane resin, the reaction of the constituent ingredients comprising the same is permitted to go to completion while left in the mold for from about 2 to 7 hours (depending on the thickness of the abrasive article to be produced) and heated during this period to approximately 200 F.

As above indicated, it is ordinarily very much preferred to include in the resin a small amount of an appropriate blowing agent effective to produce a very large number of minute cells or voids throughout the body of the finished article. Depending upon the particular blowing agent employed, as discussed more in detail below,

it may be desirable to heat the material while enclosed.

Within the mold and prior to setting of the resin to activate such agent. The mold will ordinarily have been removed from the turntable at this stage since it is usually not desired to centrifuge the same during performance of the blowing operation. Such minute cells or voids in the outer abrasive region of the tool assist in permitting micro-deformation at locally overstressed points of the working face in use, such response to high unit pressures being at least somewhat due to partial collapse of free volume therein. Furthermore, the interstices between the abrasive grains are relatively open so that the grains exposed at the working face of the tool instead of being substantially solidly embedded in individual sockets have their cutting edges much more fully exposed for active work than has been the case in prior practice. When polyurethane is employed as the bonding agent, such preferred resin evidences an unexpectedly strong bond to the abrasive grains effective to secure the latter even under severe working conditions and despite the fact that the bonding resin may contact only certain portions of the individual abrasive grains rather than substantially completely solidly embedding the latter. Excessive wear and damage to the abrading tool are likewise somewhat minimized by the inner resin or plastic central portion of the tool which serves to support the relatively more rigid outer abrasive portion (which itself is capable of local micro-deflection) in a manner to absorb violent shock and stresses which may be encountered when the latter engages the work. In some cases, however, it is preferred to remove the inner non-abrasive portion of the wheel and quite frequently at least the extreme inner portion may thus be removed to adapt the wheel to various sizes of arbors or the like.

As shown in FIGS. 5 and 6, a rotary abrading wheel produced in accordance with the invention may ordinarily be of the usual cylindrical form and provided with a central arbor hole 17 formed by stud 7 within the mold or drilled to a larger diameter as may be desired. Various metal hubs and the like may be placed within the mold and thereby included as a part of the finished article. As shown in FIGS. 5 and 6, the wheel will comprise a radially outer circumferential portion 18 ordinarily of substantial width having a large number of abrasive grains 19 slightly spaced apart out of contact with each other by the cellular resin, and an inner non-abrasive portion 20 of the resin body having a multitude of small cells therein. For purposes of clarity, such cells in both the radially inner and outer portions of the Wheel are not shown in these figures, but their relative size and disposition are indicated in certain other figures of the drawing. The side or end faces 21 and 22 of the wheel will ordinarily have an integral substantially imperforate resin skin formed thereon in the molding operation, and this skin adds appreciably to the strength of the Wheel. If desired, however, thin annular face plates of sheet metal, strong paper, cardboard or plastic may be molded and bonded to the respective end faces of the wheel.

Instead of separately introducing into the mold the binder resin and the discrete abrasive elements, these components may themselves be intermixed in advance so as to be simultaneously supplied to the mold in the same manner as above described. While it is desirable that the abrasive granules be easily wetted by the liquid resin composition and that the components be well and uniformly intermixed to the extent feasible, the uniform disposition of the abrasive grains in the final product is not dependent thereon but is obtained by means of the centrifuging and foaming steps above described. The amount of resin-abrasive mixture delivered to the mold, while only partially filling the same, will ordinarily be selected to be suificient completely to fill the mold upon expansion of the resin-abrasive body as a result of the blowing operation. The blowing operation takes place at atmospheric pressure (the mold being centrally vented), and a small amount of flash may be produced which can easily be trimmed from the finished article. In some cases, when an especially strong blowing action is obtained, centrifuging may be continued, although usually at reduced speed, during the blowing operation in order to obtain the desired uniform spacing of the individual grains while at the same time limiting such spacing to not more than approximately one grain diameter. It will be seen that there are accordingly a number of factors which maybe utilized in regulating the manufacture of the new abrasive tool including the speed of centrifuging, the viscosity of the resin binder and the activity of the blowing agent, for example.

Now referring more particularly to FIGS. 7, 8 and 9 of the drawing, FIG. 7 illustrates in much magnified, some what diagrammatic form the placement of the abrasive grains 19 in the outer circumferential portion of the mold as a result of the centrifuging operation, such grains being concentrated into substantially contacting. relationship with the interstices therebetween filled with the binder resin 23, although as above indicated some degree of foaming may already be taking place. Upon further foaming of the binder resin, ordinarily after cessation of effective centrifuging and after such resin has substantially increased in viscosity, the abrasive grains 19 are moved apart thereby as generally indicated in FIG. 8, the individual grains being uniformly spaced apart preferably not more than about one grain diameter, on the average. The resin is then set in this condition and, while forming a rather complex structure as viewed under the microscope, may nevertheless properly be described as comprising relatively thin coatings on the grain surfaces bonded to and holding the latter end interconnected by heavy web portions 24 having cells or voids therebetween, and sometimes also apparently smaller bubbles or cells formed within such web portions themselves. Such webs are of considerably greater dimensional thickness than webs which are produced when the same resin is caused to foam freely under atmospheric pressure without employment of the centrifuging step. Such heavy webs are of considerable advantagein the finished article since they play a large part in limiting the stubborn local deformation of 7 the resin body to the required micro amounts needed to permit readjustment of an individual protruding grain and absorb the excess energy of the force imposed thereon when such protruding grain is brought into engagement with the work surface under grinding pressure.

Using either of the centrifuging methods above described, a uniform circumferential region of densely concentrated abrasive material is soon produced which may be observed through a transparent cover plate, and as the blowing operation proceeds this annular circumferential abrasive region may be seen to widen appreciably in a radially inward direction due to radially inward displacement of the abrasive granules resulting from the formation of gas pockets or cells therebetween. The width of such region or band of abrasive concentration may in some cases even be doubled due to such action of the blowing compound although in many cases the effect will be very considerably less. Of course, the radially inner non-abrasive portion of the resin or plastic body will likewise simultaneously expand radially inwardly due both to such expansion of the outer circumferential abrasive bearing portion and also the formation of cells or bubbles within the portion 20 itself. As indicated in FIG. 9, such cells or bubbles 25 are normally both smaller and have thinner webs separating the same than is the case with the cells which assist in spacing the abrasive grains 19 (FIG. 8). It will be noted from the foregoing that centrifugal force is employed initially to distribute the abrasive grains in an annular region in uniform (substantially contacting) relation to each other with liquid resin in the interstices therebetween, the excess resin being separated from the grains in a radially inward direction by direct displacement. The resin is not set, however, until after the grains have been moved slightly out of contact with each other by means of the foaming operation. When the entire mass is expanded by means of the blowing operation, it is interesting and important to note that no differential effect or result is obtained in the abrasive annulus. In other words, a radial cross-section through such abrasive annulus shows the abrasive grains to be uniformly distributed and spaced not only circumferentially of the tool but also radially of such abrasive annular portion, the consequence being that the grinding characteristics of the tool in use are not appreciably altered as the diameter of the tool is reduced through repeated dressings. It is desirable that the resin be at least preliminarily gelled or set as promptly as possible after the abrasive grains have thus been properly uniformly slightly spaced, such prompt gelation cooperating with the cells in the resin body to prevent settling of the abrasive content under influence of gravity following cessation of centrifuging. The grinding tool produced in this manner will have a maximum number of cutting points exposed at the working face of the tool consistent with the requirements that the abrasive grains be slightly uniformly spaced for individual micro-adjustment and absorption of stress. Such uniform spacing of the grains is obtained in accordance with the invention inasmuch as the foaming may continue at atmospheric pressure until the resin gels sufiiciently to inhibit and finally stop the foaming. At this point, the forces producing the foaming and the forces resisting the foaming, i.e. the gelation or increased viscosity of the resin and, optionally, continued rotation of the centrifuge, will be in balance and further foaming will cease.

One of the most difficult problems encountered in the manufacture of conventional grind-ing wheels is the obtaining of uniformity from wheel to wheel within specific grades, such wheels being manufactured with dry granular materials and the compacting of such a mass into a particular shape and size being hard to reproduce. Because of the unique manufacturing methods above described and the materials employed, exceptional uniformity of wheel quality is an inherent feature of the present invention, and consequently less frequent adjustment is required to maintain close tolerances in production operations.

Abrasive material The type, grit size and amount of abrasive may be varied to produce a wide variety of useful products. All commonly available abrasive grains are adapted to be employed in the articles and manufacturing methods of this invention. In each particular case, however, it is necessary to take into consideration the size, shape, purity, and other aspects of the materials employed to ensure obtaining the desired dense concentration of the abrasive grains in the working portion of the abrading tool, the exact amount of the abrasive grain to be employed being directly determined by the bulk density (grams/ unit volume obtained by free fall) for the specific grade, size, shape, etc., of abrasive being employed. The term bulk or pack density of abrasive grains is well known and understood in the art, and figures are available for all of the common abrasive grains. The term is defined by The Grinding Institute as weight in air of a given volume of the permeable material (including both permeable and impermeable voids normal to the material) expressed in grams per cubic centimeter. For the purposes of the present invention, I very much prefer that grinding tools in accordance therewith have an abrasive content equal to form about 75% to about of the bulk or pack density of the particular abrasive employed. The abrasive grains should constitute from about 30% to about 45% of the abrasive-resin body, by volume.

Any suitable abrasive material may be utilized such as silicon carbide, aluminum oxide, emery, garnet, talc, pumice, and lime silicon dioxide, depending upon the abrading action and the resultant surface finish desired. While grit sizes of from 600 to 10 mesh may be utilized, the ordinary range will be from about 320 to about 24 mesh and most frequently from 60 to 36 mesh.

Such abrasive grains should have reasonably close size control so that they will not centrifuge differentially through the liquid media (Le. the finer grains predominately toward the center of the wheel and the coarse grains toward the outer periphery of the wheel). For example, it may be possible to centrifuge a blend of 46, 54 and 60 grit fairly uniformly but not a blend of 36 and 100 grit. Likewise, if two dilferent types of abrasives are employed as a blend, the true densities of each must be carefully considered so that the mass of the particle remains approximately the same. Thus, boron carbide having a specific gravity of 2.51 will centrifuge quite differently from aluminum oxide with a specific gravity of 3.95 even though they might both be classified as 60 grit.

The wetting ability and purity of the grain is important. The wetting properties of the abrasive material determine the speed of blending the grain with the reactive resin mixture. Acid or basic impurities on the surface of the abrasive may tend to catalyze the resin reaction. Moreover, the abrasive grain must have friability, shape, and hardness properties which are compatible with the bond-filler media in which it is embedded.

Filler material A filler such as mica (325 mesh), graphite powder (325 mesh), iron pyrites (approximately 200 mesh), silicon carbide and aluminum oxide of flour fineness, etc., may be incorporated in the resin-abrasive mix. Such filler should be so finely ground that it may be uniformly dispersed throughout the grinding wheel even when present in small quantities and should be of such specific gravity and fineness that it will essentially not centrifuge to the outer portion of the Wheel through the media of the reacting resin foam mix and abrasive slurry at the centrifuge speeds involved. That is, there should be no great difference in filler concentration in relation to the foam 9 resin at the hub and at the outer periphery of the new abrasive wheel.

The filler may desirably in certain cases help remove some of the heat of reaction by absorbing heat from the reacting mass and such filler will ordinarily assist in reducing stresses within the grinding wheel which may be present in an un'filled plastic system. The cellular structure of the cured system helps to alleviate this problem but at areas of differential coefficient of linear expansion, such as at the interface of the abrasive annulus-plastic hub portion of the grinding wheel, a filler may be useful to reduce stresses which might lead to cracking of the wheel. The filler then adds to the desirable grinding qualities of the wheel, and fillers such as graphite or mica may impart lubricating qualities to the wheels. One such as sulfur, iron pyrite, or cryolite may impart cooling qualities during grinding because they decompose or boil at or below normal grinding temperatures. Fillers such as fine silicon carbide or aluminum oxide flour may also impart additional abrasive action to the wheel and may be desirable for fine finish work.

Matrix or binding material Formula No. 1

Moles Glycerol 4.0 Adipic acid 2.5 Phthalic anhydride 0.5

Formula No. 2 Trimethylol propane 4.0 Adipic acid 2.5 Ph'th-alic anhydride 0.5

Formula No. 3 Glycerol 2.0 'Pentaerythritol 0.5 Phthalic anhydride 1.0 Sebacic acid 3.0

Formula No. 4 Trimethylol propane 3.0 Phthalic anhydride 2.0 Formula No. 5 Trimethylol propane 4 Adipic acid 1 Phthalic anhydride /2 Dimer acids /2 Wheels may thus be produced with the above formulations which comprise a substantially rigid, dimensionally stable, non-brittle, infusible, cellular foamed body made by reacting a material selected from the group or class consisting of aromatic olyesters and polyethers with a polyisocyanate to produce such rigid polyurethane.

A formulation such as that below may also be employed utilizing aliphatic polyesters and polyethers in the reaction with the polyisocyanate also to produce a crosslinked rigid thermosetting type foam:

Formula No. 6

Moles Trimethylol propane 3% Dimer acids Oxalic acid 2% Cross-linking can be obtained in a polyurethane system if one of the components of the polyester, usually the hydroxyl bearing group is trifunctional or higher. The polyester with free hydroxyl groups present can be subsequently cross-linked with a diisocyanate to form the finished polyurethane. Also, such cross-linking in a polyurethane system can be obtained if the system is a polyether, the polyether, however, being in the form of a triol or higher to cross-link with the diisocyanate; the shorter the distance between the cross-linking hydroxyl positions, the more rigid the structure obtained. Also, such crosslinking can be obtained if a linear polyester based on glycols or a polyether system containing diols is cross-linked by using a triisocyanate (i.e., triphenylmethane triisocyanate). However, in commercial practice, the first two methods of cross-linking a polyurethane system are de sirably employed to obtain such rigid foams.

Polyesters such as the above used in polyurethane formulations should have an acid number of from less than one to forty and have the following ratio range of the hydroxyl to the carboxyl groups in the resin reactants: From four hydroxyl (OH) to one carboxyl (COOH); to one hydroxyl (OH) to one carboxyl (COOH). The pre ferred ratios are from three hydroxyl (OH) to one carboxyl (COOH); to 1 /2 hydroxy (OH) to one carboxyl. The excess of hydroxyl groups ensures the subsequent reaction with polyisocyanate to form polyurethanes.

The dimer acids or dimerized fatty acids included in certain of the above examples of alkyd resins are dimeric polymers of unsaturated fatty acids such as: dimerized linolenic or linoleic acids. These dimer acids may be prepared by heating the methyl esters of polyunsaturated acids such as linoleic or linolenic acids at high temperatures. This is represented diagrammatically by 3 Bids- Alder reaction to form the dilinoleic acid (dibasic unsaturated acid) as follows:

A suitable one-shot polyurethane may be made by blending one of the above polyesters with the theoretical amount of slight excess of polyisocyanate, preferably toluene diisocyanate (either 2,4 or 2,6 toluene diisocyanate or mixtures thereof), to react with the excess hydroxyl groups present in the polyester. A possible reaction is shown in FIGURE No. 1, using toluene diisocyanate and a polyester as indicated in Formula 1.

The polyisocyanate employed in preparing the reactant foaming compositions may be used either with or without one or more thermoplastic polymeric resin additives, the latter serving to stabilize the foam during the reaction. Ethyl cellulose has been a particularly effective additive in this respect and the preferred range of addition would be from 0 to 8 parts of ethyl cellulose per parts of toluene diisocyanate, by weight.

Heat resistance of the above formulations may be improved by adding polymethylol phenyls in the reactant compositions or mixtures for producing cellular plastics.

The preferred method of producing foam in the above systems is to incorporate from 0.1% to 3.0% H 0 by weight into the alkyd resin. The water may be incorporated as liquid water; however, other means may be employed, such as one or more metallic salt hydrates. Wetting agents such as glycerol monoricinoleate may also be incorporated to aid in the uniform dispersion of the water into the alkyd resin. The reaction between the polyisocyanate and water forms an intermediate product, carbamic acid, which decomposes to give a primary amine and carbon dioxide gas, the blowing agent.

A typical system would be a polyester of the type indicated in Formula 1, with a hydroxyl number of 450-470, a water content of 0.1 to 1.0% and a viscosity of 120,000 to 160,000 cps at 70 F. The polyester may have additives such as 2,4,6 trimethylol allyloxy benzene included to improve heat resistance. This composes one component of the one-shot system. The other component is composed of a diisocyanate, preferably toluene diisocyanate (either 2,4 or 2,6 toluene diisocyanate or combinations thereof) which can be blended with from to 8 parts by weight of ethyl cellulose per 100 parts toluene diisocyanate. In machine mixing, the polyester, or resin component is normally heated to a suitable temperature, from 100 to 150 F., so that the resin can be pumped and dispensed more readily. The toluene diisocyanate component may have an initial viscosity of from 1 to 5000 cps. at 70 F. depending upon the amount and type of ethyl cellulose present. A filler such as mica above noted then may be incorporated.

The ingredients may usually he mixed at room temperature although they may, if desired, be preheated to reduce viscosity and increase the rate of reaction. They may be mixed for about one minute and then poured into the spinning mold, the latter operation requiring about 30 seconds and the centrifuging about 45 to 240 seconds. The duration of centrifuging at speed depends upon several factors. Such centrifuging must be sufiiciently long and the speed sufficiently high to cause the abrasive grains to be uniformly concentrated in the outer periphery of the wheel. The duration of centrifuging must also be sufficiently long to allow polymer and viscosity build-up so that the abrasive annulus will not slump seriously due to gravity when effective centrifuging ceases. However, the centrifuging should be stopped while foaming action is still sufficient to separate the abrasive grains and to cause the reacting, foaming resin to travel radially inwardly and complete the dimensions of the mold. The speed of centrifuging may vary from several hundred r.p.m, to several thousand rpim. depending primarily upon the diameter of the wheel produced. The turntable or spinning mold may then be stopped and the foaming operation proceeds for approximately 10 minutes to fill the central portion of the mold and to widen the outer circumferential abrasive region radially inwardly through uniform spreading of the abrasive elements slightly apart. Another 10 minutes may be required for initial setting, and then minutes or more for final setting.

Foam may, of course, be generated in known manner in various types of resins by whipping or heating, or by inclusion of soluble granules which are subsequently dissolved out, or by introduction of gases under pressure. The term foam as herein employed is intended to include cellular structures without regard to the particular manner in which such cells may be formed.

To manufacture a preferred abrading wheel, 162 grams of an alkyd resin, such as given in Formula 1, is mixed with 138 grams of toluene -2,4 diisocyanate for one minute. An abrasive material such as 330 grams of 36 grit fused aluminum oxide may be mixed into the above alkyd-diisocyanate mixture. The foregoing mixture is immediately placed into the aforedescribed mold and then rotated at about 3000 rpm. for one minute. The mold is then placed into an oven at approximately 250 F. for two hours. The mold may then be removed from the oven and cooled before the finished, foamed abrading wheel is removed. The finished wheel should weigh 520 grams. The difference in the original weight of materials and the article weight is accounted for by the cling in the mixing cup. The cling material and the material in the mold each contain the same proportion of grit and plastic as the mix.

Satisfactory wheels may also be made by varying the foregoing procedure; for example, the abrasive material may be premixed into the alkyd resin or diisocyanate portion of the mixture. The alkyd resin may be varied as to the nature of its chemical components as given, for example, in Formulas 2, 3, 4 and 6. Abrasive type and grit size may also be varied to produce the desired type of abrading action in the finished wheel. The toluene diisocyanate portion of the foregoing mixture may be varied by using mixtures of toluene -2,4 diisocyanate with toluene -2,6- diisocyanate.

Another formulation for making, for example, 7'' OD. by /2" wheels using a resin sold by Nopco Chemical Company under the trade name Lockfoam would be as follows:

162 grams A625R Lockfoam resin manufactured by The Nopco Chemical Company 138 grams A-625-C foaming agent also manufactured by The Nopco Chemical Company 330 grams abrasive grit as the aforementioned aluminum oxide, for example The foregoing materials may be mixed in the order given above, at a temperature of 70 F., to start. The resin and foaming agent may be mixed for seconds and then the abrasive grit is added and mixed for an additional 45 seconds. This mixture may then be placed in a mold having a volume larger than that of the mixture, such mold being made so that it is open to the atmospheric pressure. The mold is then rotated to centrifuge the contents for 45 seconds at about 2800' rpm. While still in the mold, the article is cured at about 200 F. for 1% hours, after which it is cooled to room temperature before the mold is opened. The weight of the final wheel is 520 grams.

Wheel composition and density The composition of the abrasive annulus 13 can be obtained by burn-out tests utilizing certain procedures to obtain the composition breakdown. The density and volume of the grinding wheel can be accurately determined by means of ASTM D792- (Specific gravity by water displacement) if the grinding wheel is essentially closed cell and does not absorb water rapidly, this latter feature being important in grinding wheels which are often run with Coolants. If fluids are absorbed by the wheel, the latter may tend to become out of balance. In such tests, sections of wheels of known weight and volume are placed in a crucible and fired for at least one hour in an oven maintained at 1300 F. During this period, all organic bond material is driven off as volatile matter. Also, if a filler is present which either boils or decomposes below the oven temperature, it will be driven off with the bond material. If not, the filler will remain in the crucible with the abrasive which is unchanged at this temperature. Examples of fillers that are not affected at this temperature are mica and graphite. Since these are present as very fine particles, they can be separated from the heavier or larger abrasive particles by washing in a manner similar to that employed in ore separating processes. An example of a filler which is volatilized in the oven at such'temperature is sulfur which boils at 832 F. This must be subsequently separated from the bond by chemical analysis, using a fresh sample from the same grinding wheel. By weighing the remaining contents after burn-out, with an analytical balance and also after Wash-out and drying, a very accurate determination of composition by weight can be made with excellent duplication.

However, weight composition does not reveal the complete story of the grinding wheel. Volume composition, which includes air or gas space, sometimes referred to as porosity in the grinding industry, is a very significant factor. To obtain volumetric composition, the densities of the abrasive, bond and filler can be ascertained. As an example:

gmJcc. Aluminum oxide abrasive density 3.95 Polyurethane bond density 1.20 Mica density 2.84

The aluminum oxide abrasive density can vary from 3.90 to 3.97 gm./cc., depending on friability, but 3.95 gut/cc. is a good over-all average. The 1.20 gin/cc.

density for the non-foamed polyurethane bond system can be obtained from suppliers of the resins. This density may, however, vary somewhat for other rigid polyurethanes employed, but such variance is not particularly significant. Since the total volume and the volume occupied by the constituents of the grinding wheel can be ascertained, the volume occupied by the air space can be determined by the difference. The over-all density is determined as above set forth and the abrasive density can be determined as simply the over-all density times the weight percent of abrasive. By using the volumetric composition, different abrasives such as silicon carbide or diamond; or different bonds, such as epoxy and phenolic; or different fillers, such as sulfur or cryolite, find utility in the wheel of the present invention. The weight make-up may change quite considerably with such different components, but the volumetric make-up will remain approximately the same. The weight make-up is important when related to a specific system; i.e., aluminum oxide, polyurethane bond, mica filler; and is alsoincluded to help define the product of the present invention.

The following examples 1-5 indicate the results of this analysis:

14 hours at 200 F. The wheels, as indicated, were subjected to burn-out tests by splitting the abrasive annulus on the parting line 26 in FIG. 5 and then running the burnout tests separately on each portion.

The above five examples are indicative of the compositions obtained in a seven inch diameter grinding wheel. Larger wheels must generally be made by more rapid means because much larger amounts of material must be handled in the same period of time. In example 6, set forth below, a 24 O.D. x %1" thick wheel was employed, such wheel being made by blending abrasive with the reactant mix in two separate cups and filling by pouring into a rotating mold, stopping and inserting the vented core into the filled mold before going into high speed centrifuging. The results of burn-out tests at various positions in the abrasive indicate some differences in composition as the radial distance from the outer edge changes. However, it is to be noted that at identical radial distances (such as work encounters when acted upon by the grinding wheel since the grinding wheel is rotated on the same axis as that on which it was initially produced), the wheel has excellent uniformity.

Mix in Mold, 8 m. Wt. Percent in Centri- Vol. Percent in Centii- Abrasive Annulus,

Wt. fuged Abrasive Annulus Vol. fnged Abrasive Annulus gm. cc. Percent Percent Abrasive Exin Mold Outer Inner in Mold Outer Inner Overall Abrasive olon XW fiO Bond Filler Mix Avg. Half Half Mix Avg. Half Half Density Density Grit Example 1 313 130. 4 17. 6

Abrasive 57 9 74. 36 74. 36 74. 37 26. 0 32. 32. 22. 35. 6 32. 42 32. 43 3. 2. 0 1.89 1.90 36. 4 33.20 33.12

In the Examples 15 above, the resin system used is EXAMPLE 6 a polyestenbased one-shot polyurethane such as that previously set forth. The filler (325 mesh white waterground mica) was preblended with the toluene diisocyanate component of the system so that the foaming resin system contained filler as it was dispensed from the mixing machine. The foam system in each case is a normal 25 pound per cubic foot free foam density rigid polyurethane. The resin is preheated to 130 F. The toluene diisocyanate component was thinned to obtain a toluene isocyanate with 2 parts ethyl cellulose per '100 parts T.D.I. The filler was then preblended into the above solution and the blend dispersed at a temperature of approximately 80 F. The mix of mica-filled foam reactants was dispensed into a cup and the abrasive added to the top and blended into a uniform mixture by mixing with a loop-type mixer for 15 seconds. The uniform blend was then dispensed into a steel mold of dimensions 7" O.D. x 1 /4 ID. x /2 thick maintained at a temperature of 150 F. The top was placed on the mold secured with a nut and centrifuging is started 75 seconds after the foam shot was taken. The mold was rotated at 2200 r.p.m. for 45 seconds, then allowed to coast down to rest (about 20 seconds). The mold was then removed from the centrifuge and placed into an oven for three Radial distance from outer edge (in.) 0. 375 1.13 1.75 2. 25 3.13 Over-all density (gm. cc.) 2. 48 2. 45 2. 37 2. 30 2. 23 Abrasive density (gm. cc.) 1.83 1. 81 1. 74 1. 68 1. 61 Composition, Weight Percent:

Abrasive 73.8 74. 0 73. 3 72. 8 72. 1 Filler (325 mesh mica) 3. 5 3. O 3.0 3. 0 3. 0 Bond rigid polyurethane 22. 7 23. O 23. 7 24. 2 24. 8 Composition, Volume Percent:

Abrasive 46. 9 4'3. 4 44. 6 43. 0 41. 3 Filler (325 mesh mica) 3. 0 2. 6 2. 5 2. 4 2. 4 Bond-rigid polyurethane 47. 0 46. 9 46. 8 46. 4 46. 2 Air 3. l 4.1 6.1 8. 2 10.1

In the above example, 1916 grams of the rigid polyurethane reactants were mixed with 259 grams of 325 mesh mica and with 3325 grams of the abrasive and were agitated with a loop-type mixing blade until the materials were well blended (approximately 15 seconds). The ingredients were then poured into a mold rotating at approximately 550 rpm. In approximately 75 seconds from the start of the first shot of the foam before the ingredients had been poured into the mold, the centrifuging was stopped and the mold plug secured. Even with the plug in place, there is adequate central venting of the mold, however. The centrifuge was then spun at 800 rpm. for approximately seconds. The mold was then re- EXAMPLE 7 20" 0.1). X 2" thick] Radial distance from outer edge (iu.). 0.375 1.1 Over-all density (gin/cc.) 2. 51 2. 4 Abrasive density (gmjec l 1. 1 Composition, V. eight Pe Abrasive Filler (325 mesh mic-a) Bond (rigid polyurethane) Composition, Volume Percent:

Abrasive Filler (325 mesh mica) In the above example, 6147 grams of the rigid polyurethane reactants were mixed with 829 grams of 325 mesh white waterground mica along with 10,708 grams of 60 grit size abrasive and dispensed into a mold rotating at 700 rpm. The mixing was accomplished by four blades rotating at the same speed as the mold plus an air nozzle to keep the material from building up on the sides of the mixing chamber. The mold was then stopped, cored and then rotated at 1060 rpm. for 120 seconds. The wheel was then cured for seven hours at 200 F. with the final weight of the wheel being 17,450 grams.

Another excellent polyurethane system can be obtained using polyether as a base. Examples of usable polyethers include reactive polyglycols. The preferred type of polyether is one which is trifunctional or higher (i.e. triols, pentols, hexols). A good system for use with grinding wheels of the present invention is called a quasi-system and is shown as follows as weight percent.

Part A Percent Polyether 20 Toluene diisocyanate 78 Catalyst 0.2 Emulsifier 1.8

Part B Percent Polyether 99.3 I Water 0.1 Catalyst 0.6

Parts A and B can be blended in a mixing machine in the ratio of 4 parts A to 3 parts B.

For the preferred grinding wheel of the present invention which will make a very precise cut at predetermined high pressure, it is important to select a rigid, infusible, dimensionally stable, cross-link resin for the foam. Crosslinking of this type can be obtained in a polyurethane system if:

(1) One of the components of the polyester, usually the hydroxyl bearing group, is trifunctional or higher. The polyester, with free hydroxyl groups present, can be subsequently cross-linked with a diisocyanate to form the finished polyurethane.

(2) If the system is a olyether, the polyether must be in the form of a triol or higher to cross-link with a diisocyanate. The shorter the distance between the crosslinking hydroxyl positions, the more rigid the structure.

(3) A linear polyester based on glycols or a polyether system containing diols could possibly be cross-linked by using a triisocyanate (i.e. triphenylmethane triisocyanate) but in commercial practice systems 1 and 2 are usually used to obtain rigid foams.

As previously stated, other resins than the polyurethanes mentioned are suitable for use with the grinding Wheels of the present invention. One example thereof is an epoxy resin mixture which will produce a foam system, such as the following:

Amount Purpose (grams) Ingredient Resin 14. 7 EPON 828-epoxyresin (Shell Chemical.)

Do 31. 3 EP 1004-epxy resin (Shell Chemica Filler 26. 0 Mica-325 mesh white watergrouud. Diluent-heat 6. 0 To1uenetechnical grade.

absorber. Curing agent. 11. O Diethylene triamine. Wetting agent. 4 drops Tween 20-polyoxyethylenc (20) sorhitau monolaurate. Blowing agent O. 5 Argugonium earbonatepowdered, purie Abrasive 334. 0 Aluminum oxide-60 grit.

Procedure:

(1) Preblend 147.7 grams EPON 828, 31.3 grams- EPON 1004, 26 grams mica, 4 drops Tween 20 and 6 grams xylene and heat to 170 F.

(2) Add 11 grams diethylene triamine and blend with a high speed loop-type mixer for seconds.

(3) Add 334 grams aluminum oxide, 60 grit, at 170 F. and blend for additional 10 seconds.

(4) Add 0.5 grams powdered ammonium carbonate and blend additional 15 seconds until finely dispersed.

(5) Add above mixture to mold of dimensions 7' OD. x 1% ID. x 0.500" thick (305.3 cc.), close mold and centrifuge at 2200 r.p.m. for 60 seconds.

(6) Allow mold to coast to a stop (20 seconds), then remove mold and place in oven to cure for three hours at 200 F.

(7) Final weight-520 grams.

EPON resin 828 has an epoxide equivalent of 175210, molecular weight of 350400. EPON resin 1004 has an epoxide equivalent of 8701025 and a. molecular weight of 1400. Other epoxy resins such as EPON 834 (epoxide equivalent 225-290, molecular weight 450) and EPON 1001 (epoxide equivalent 45 0-525, molecular weight 9001000) may be used. Diethylene triamine is the curing agent, but curing agents such as metaphenylene diamine may be included to improve strength, heat and chemical resistance of the bond. Mica is the filler, but other fillers previously described may be employed. Toluene is the solvent-diluent used to modify and control the foaming process by absorbing excessive heat of reaction. Tween 20 is used as a wetting agent to provide a fine and uniform dispersion of gas bubbles. Ammonium carbonate is the blowing agent, but various other blowing agents can be incorporated, such as Celogen (P, P oxybis [benzenesulfonyl hydrazide1), nitroso compounds, azo compounds, hydrazides, etc. Thus, the resin-catalyst wetting agent-blowing agent system may be varied to obtain a proper cross-linked system for abrasive wheels.

Phenolic foams have also been found suitable for 1 production of grinding wheels in accordance with the present invention.

Part A (parts by weight) BRLA 2761 BRLA 2760 20 Isopropyl ether 6.6

Tween No. 40 1 Part B (parts by weight) Water (as ice) 50 Sulfuric acid 68 Baum 50 Phosphoric acid 7 Part A is prepared as follows: The Tween 40 is dispersed in the isopropyl ether and then, with continuous stirring, this blend is mixed into the blend of BRLA 2761 and BRLA 2760. Part B is prepared by adding the sulfuric acid very slowly to ice. When this addition is complete and well stirred, the phosphoric acid may be added and mixed well. The ratio of components is from 8 parts of Part B, to 92 parts of Part A, to 16 parts of Part B, to 84 parts of Part A.

BRLA 2761 and BRLA 2760 are liquid phenolic resins produced by the Union Carbide Plastics Company. Each is composed of an incompletely condensed resin made by the interaction of phenol and formaldehyde. The water miscibility of BRLA 2761 is 205%, that is, 2.05 parts by volume of water mixed with 1 part by volume of BRLA 2761 will still form a solution. Above this point, additional water will cause an emulsion to be formed. The water miscibility of BRLA 2760 is 195%. When these resins are mixed with catalyst and blowing agent, an exothermic reaction of further condensation causes liberation of the gas by the blowing agent. The increasing viscosity of the mix prevents escape of this gas and hence the mass expands until the resin sets up. Tween 40 (polyethylene sorbitan mono palmitate) is a wetting agent which helps to provide a fine and uniform dispersion of gas bubbles.

The abrasive will ordinarily preferably be premixed with one or more of the resin constituents prior to charging into the mold and such mold may be caused to rotate during the charging process at a rate sufficient to move the mixture radially outwardly therein, such rate being increased as soon as charging is completed in order to cause the abrasive to become concentrated in the outer annular region. In the case of the polyurethane reactant constituents, initiation of the reaction appears to result in a temporary reduction in viscosity which facilitates such centrifuging, followed, of course, very promptly by increase in viscosity, as the reaction progresses. The abrasive will ordinarily be incorporated in the resin before appreciable foaming commences but if the resin begins to foam While charging the mold the centrifuging of the latter serves to inhibit further foaming while rotating at high speed, all or most of the gas which may be generated during this stage being centrally vented as the cells are compressed and collapsed.

FIGURES 12, 13 and 14 of the drawing illustrate in semi-diagrammatic fashion the principal operational stages of the preferred mode of manufacture. Thus, in FIG. 12 the resin-abrasive mixture 27 is shown in the rotating mold 28 forming an annular body having a radially inner surface 29, with the remainder of the mold being empty except for central stud 8. When the rate of rotation is now increased, the abrasive constituent is centrifuged to a radially outer region 30, with the portion of the resin 31 which is now substantially abrasive-free lying immediately radially inwardly therefrom. When effective centrifuging is terminated (the mold ordinarily being stopped), the abrasive containing region now widens inwardly as shown at 30' in FIG. 14, the inner line of demarcation 32 between the abrasive and non-abrasive regions thus moving radially inwardly, due to generation of a multitude of small cells in the resin in the interstices between the abrasive grains, and the abrasivefree region 31 likewise expands radially inwardly normally completely to fill the mold as shown at 31' in FIG. 14 due both to such expansion of region 30 and also to generation of cells within the region 31' itself.

It is much preferred to centrifuge about a substantially vertical axis since this simplifies the problem of maintaining a uniform wheel structure without slumping, especially of the radially outer abrasive region, when effective centrifuging is stopped and widening of the ab rasive containing region takes place in the highly viscous but not yet set resin. The degree of hardness or rigidity to which the resin is then set will depend on the characteristics desired, the method of this invention being useful in the production of a wide range of abrasive tools, but when producing a true grinding wheel the resin will be set to substantial rigidity with only a very slight stubborn yielding capability.

In the preferred mode of operation, the granular abrasive may be concentrated by centrifugal force in the outer annular region of the mold to a degree exceeding the pack density of the particular abrasive grains, the

liquid resin and centrifugal force cooperating to achieve such concentration. The foaming back of such concentrated abrasive region, i.e. the expanding of such region by generation of minute cells between the grains comprising the same, will preferably be to an extent to space such grains uniformly apart to occupy a volume only slightly greater than that occupied by such grains at pack density. In this way maximum grinding efiiciency can be obtained with provision for slight individual movement and consequent impact absorption when each exposed grain engages the work in use. In some cases, however, somewhat greater spacing of the grains may be desired, although seldom to an extent affording a grain density of less than of the pack density for the particular abrasive material.

It has been found to be very advantageous to produce an abrasive wheel by providing a mixture of liquid foamable binder resin, fine particulate filling material uniformly distributed therein, and granular abrasive in a rotatable circular mold. The mold is rotated at a speed sufiicient t-o centrifuge such granular material to a uniform outer annular region in such mold but insufficient appreciably to affect such uniform distribution of such fine filling material, the size and the mass of the latter making it much less subject to the action of centrifugal force thereon in such resin. The resin is foamed to produce cells therein between the grains of such abrasive to space such grains apart, and the resin is then set to the desired degree of rigidity. The cells produced in the radially outer annular region including such abrasive are of larger size than in the radially inner non-abrasive containing region of such resin, and the resin webs between the cells are of greater thickness in such radially outer region than in such radially inner region, the abrasive grains being supported in such thicker webstructure.

Such fine filling materials may be mica, silicon carbide, aluminum oxide, precipitated alumina, graphite, and many others. Certain of them, such as mica, may serve to carry moisture throughout the polyurethane react-ant mixture to ensure uniform foaming of the latter, but in general they serve to strengthen the wheel and reduce stress concentration at the line of demarcation between the outer abrasive region and inner non-abrasive region of the Wheel. Many of them, of which silicon carbide is an excellent example, have heat sink qualities, i.e. they serve to absorb heat generated exothermically by the reaction of the resin constituents and thereby prevent heat deterioration of the radially inner portions of such wheel during manufacture. Wheel side face shrinkage inwardly of such outer abrasive region may also be avoided when such filler materials are employed.

FIGS. 13 and 14 of the drawing are, of course, semidiagrammatic in nature and it should be appreciated that only a relatively small inward expansion of the concentrated abrasive region 30 will be required to produce quite a dramatic difference in the operating characteristics of the finished wheel. Thus, for example, if the mold has an inner diameter of seven inches and the abrasive region 30 (FIG. 13) is approximately one and one-half inches in width, such latter region need be expanded only on the order of about one-sixteenth of an inch to achieve such improvement. The exact degree of expansion preferred will depend somewhat on the type, shape and size of the particular abrasive grains employed.

The new method of this invention has manifold advantages in addition to the production of a novel abrasive to-ol having exceptional working characteristics; such method is inexpensive, requiring quite low capitalization, it is unusually rapid, permitting large scale production line manufacture, and it enables consistant production of a uniform, precisely reproducible product.

"Other modes of applying the principle of the invention may be employed, change being made as regards the de tails described, provided the features stated in any of the following claims or the equivalent of such be employed.

I therefore particularly point out and distinctly claim as my invention:

1. The method of forming an abrasive article which comprises mixing discrete abrasive particles in a foamable binder medium, centrifuging the resultant mixture in a rotatable mold to concentrate said particles in a radially outer local region of said mold and then foaming said centrifuged mixture to produce cells to space substantially all of said concentrated particles only slightly uniformly apart, and setting said binder.

2. The method of claim .1, wherein a quantity of said binder is employed in excess of that remaining in the interstices between said abrasive particles following centrifuging and said foaming acts thus to space apart said particles by said cells a distance not exceed-ing substantially one particle diameter, excess gas generated by said foaming operation is vented centrally of said mold, and said binder is set to substantially rigid condition, so as to afford clearance for only slight relative movement of said particles exposed at the Workin g face of said brasive article engaging the work under grinding conditions in use.

3. The method of producing an abrasive article from a mixture of discrete abrasive elements and a foamable hardenable binder material, which comprises preliminarily concentrating said abrasive elements into close uniform substantially contacting relationship in a quantity of said binder material in excess of that required to fill the interstices between said concentrated abrasive elements, causing foaming of said binder material to produce cells to space said abrasive elements only slightly apart, and then hardening said binder material.

4. The method of producing an abrasive wheel which comprises incorporating discrete abrasive particles in a foamable liquid binder to form a liquid abrasive-binder mixture, centrifuging such resultant mixture in a mold to concentrate such abrasive particles in a uniform outer annular region with adjacent abrasive particles substantially in contact with each other and binder in the interstices therebetween, foaming such binder to generate cells in the interstices between such particles thereby to space substantially all of such particles slightly uniformly apart, ceasing effective centrifuging prior to gellation of such binder, to facilitate such foaming, and subsequently gel-ling and setting such binder.

5. The method of claim 4, wherein such binder has a tendency to foam during performance of such centrifuging step but the force produced by the centrifugal action serves to inhibit such tendency until such centrifugal action is substantially reduced by lessening the speed of rotation of such mold.

, 6. The method of claim 4, wherein the viscosity of such binder is caused to increase by continuing polymerization toward gell-ation during performance of such centrifuging step, and such foaming following cessation of effective centrifuging accordingly occurs in a more viscous medium and is therefore effective to move such abrasive particles only slightly apart.

7. The method of claim 4, wherein such liquid binder is a reactive mixture of polyurethane constituents which increases in viscosity due to progress of such reaction during such centrifuging operation, and wherein such binder is set to substantially rigid condition following such centrifuging and foaming operations.

8. The method of claim 4, wherein such foaming produces a multitude of closed cells in such binder.

9. The method of producing an abrasive wheel which comprises incorporating discrete abrasive particles in a foamable liquid binder to form a liquid abrasive-binder mixture, centrifuging such resultant mixture in a circular mold only partially filled thereby to concentrate such abrasive particles in a uniform outer annular region with adjacent abrasive particles substantially in contact with each other and binder in the interstices therebetween, foaming such binder to generate cells in the interstices between such particles thereby to space substantially all of such particles slightly uniformly apart and to expand the body of binder and abrasive in a radially inward direction, and subsequently gelling such binder.

10. The method of claim 9, including venting such mold centrally to facilitate such radially inward expansron.

11. The method of producing a rotary grinding wheel which comprises reacting a material selected from the class consisting of aromatic polyesters and aromatic polyethers with a polyisocyanate to produce essentially rigid polyurethane, producing foaming of such reaction mixture, incorporating granular abrasive in such mixture, and centrifuging such mixture and abrasive in a mold to concentrate the latter in the outer region of such mold with such grains substantially in contact with one another; stopping such centrifuging and further foam-ing such .polyurethane in unpressurized condition completely to fill such mold and through generation of cells therein to move such grains slightly apart out of substantial contact with one another, and thereafter setting such polyurethane to such essentially rigid conditions, such set polyurethane nevertheless being capable of a small degree of stubborn local yielding action sufiicient to allow individual grains exposed at the outer working facet to readjust their positions relative to adjacent grains to prevent deep scoring of the Work or dislodgment of such grains in forcible engagement with such work.

12. The method of producing a grind-ing wheel which comprises providing a mixture of liquid foamable polyurethane reactant constituents and granular abrasive particles in a rotatable circular mold, rotating such mold at a speed sufficient to centrifuge such particles to a uniform outer annular region in such mold with such particles in substantially contacting relationship, increasing the viscosity of the liquid polyurethane while inhibiting foaming thereof by such centrifuging, then ceasing effective centrifuging and generating a multitude of small closed cells in such now viscous polyurethane in the interstices between such substantially contacting particles to move the latter very slightly uniformly apart, such viscosity and cell generation acting to support such annular arrangement of such particles against deleterious slumping under the influence of gravity while such annular arrangement is caused to exp-and in a direction radially inwardly of such mold by action of such cell generation, and then setting such polyurethane to substanially rigid condition, such set polyurethane nevertheless being capable of a small degree of stubborn local yielding action sufficient to allow individual particles exposed at the outer working face of the wheel to readjust their positions relative to adjacent particles to prevent deep scoring of the work or premature dislodgment of such particles in forcible engagement with such work.

13. The method of producing an abrasive wheel which comprises providing a mixture of liquid foamable binder resin and granular abrasive in a rotatable circular mold, rotating such mold at a speed sufficient to centrifuge such particles to a uniform outer annular region in such mold with such particles in substantially contacting relationship, substantially increasing the viscosity of such resin while continuing such centrifuging, then ceasing effective centrifuging and generating a multitude of cells in such now viscous resin in the interstices between such substantially contacting particles to move the latter very slightly uniformly apart, such viscosity and cell generation acting to support such particles against deleterious slumping under the influence of gravity while such annular region of particles is caused to expand in a direction radially inwardly of such mold by action of such cell generation, and then setting such resin to the desired degree of rigidity.

14. The method of claim 13, wherein such centrifuging is performed about the vertical axis of such circular mold.

15. The method of producing an abrasive article which comprises providing a mixture of liquid foamable binder resin and abrasive particles in a rotatable mold, rotating such mold at a speed sufficient to concentrate such particles in a local region of such mold, increasing the viscosity of such binder while continuing such centrifuging, generating cells in the now substantially more viscous binder in the interstices between such particles to move the latter apart, and setting such binder to the desired degree of rigidity.

16. The method of producing an abrasive wheel which comprises providing a mixture of liquid foamable binder resin, fine particulate filler material uniformly distributed therein, and granular abrasive, in a rotatable circular mold; rotating such mold at a speed sufficient to centrifuge such granular abrasive material to a uniform outer annular region in such mold but insufficient appreciably to affect such uniform distribution of such fine filler material, the size and the mass of the latter making it much less subject to the action of centrifugal force thereon in such resin; foaming such resin to produce cells therein between the grains of such abrasive to space such grains apart; and setting such resin to a desired degree of rigidity.

17. The method of claim 16, 'wherein "a web structure is produced between the cells produced by such foaming wherein such webs are of greater thickness in such radially outer annular region including such abrasive than in the radially inner non-abrasive containing region of such resin.

18. The method of producing an abrasive article which comprises mixing discrete abrasive elements with a foamable hardenable binder material, acting on said abrasive elements to concentrate them uniformly in substantially contacting relationship in said binder material, causing foaming of said binder material to produce a multitude of very small cells effective to space said abrasive elements apart so that said spacing is only slightly more than at the pack density of the abrasive elements employed, and setting such binder material to the desired degree.

19. The method of claim 18, wherein said abrasive elements are thus concentrated in said binder material in substantially contacting relationship prior to substantial foaming of said binder material.

20. The method of claim 18, wherein such abrasive elements are concentrated in such binder material in substantially contacting relationship by action of centrifugal force prior to substantial foaming of such binder material.

21. The method of producing a molded abrasive article which comprises causing abrasive material to move through a fluid binder within a mold to concentrate said abrasive material in a localized region within said mold, forming a multitude of small gas cells within said binder including the portion thereof in the interstices between elements of said abrasive material to spread the latter apart, and solidifying the resultant composite article to form a unitary body having both a cellular relatively abrasive region and a cellular relatively non-abrasive region.

22. The method of producing an abrasive article which comprises forming a densely compacted body of discrete abrasive elements locally contacting one another having a foamable binder in the remaining interstices therebetween, causing said binder to foam to form a multitude of small gas cells in said interstices, spacing said abrasive elements apart, and solidifying the resultant composite article to form a unitary cellular abrasive body.

23. The method of producing a molded rotary tool which comprises charging discrete abrasive material and polyurethane constituents into a mold, rotating such mold to press the contents against an outer wall thereof under the influence of centrifugal force and thereby concentrate suc-h discrete abrasive in a local outer region, and thereafter appreciably expanding such outer region of abrasive concentration in a radially inward direction by generating a multitude of small cells in such polyurethane c011- stituents prior to setting of the latter within the interstices between such discrete abrasive material.

24. The method of producing an abrasive article which comprises foaming a resin binder, suspending discrete abrasive material in such foamed binder, centrifuging the resultant mixture in a mold to concentrate such abrasive material in a radially outer portion of such mold by centrifugal force, further foaming such binder effective somewhat to space apart such discrete abrasive material, and setting such binder.

25. In a method of molding an annular body of foamable plastic, the steps which comprise confining suc'h body in liquid form in a rapidly rotating circular mold centrifugally to distribute such body in annular form therein only partially filling such mold, foaming such annular plastic body to cause the same to expand in a radially inward direction only while laterally confining both sides of such expanding body, venting excess gas produced by such foaming operation in a radially inward and then generally axially outward direction at the center of such mold, and setting such plastic with the remainder of such gas forming a multiple of small cells therein, granular abrasive being intermixed with such plastic and displaced radially outwardly by the centrifugal force generated by such rotation of such mold, and a fine filler material being uniformly intermixed with such plastic and remaining thus uniformly intermixed and substantially unaffected by the centrifugal force which is thus effective to displace such solid granular material.

26. The method of claim 16, wherein the cells thus produced are of larger size in said radially outer annular region including said abrasive than in the radially inner non-abrasive containing region of said resin.

27. The method of claim 4 including the further step of removing the radially inner portion of the resultant wheel to remove the resin from this region and leave only an outer annular region containing said abrasive particles therein.

28. The method of claim 27 :wherein said abrasive particles constitute from about 30% to about 45% by volume of said outer annular region remaining after said boring operation, said region having an abrasive content of a density equal to from about to about of the pack density of the particular abrasive employed.

29. A method for the production of shaped cellular resin bonded abrasive articles which comprises introducing liquid resin binder material and abrasive grains into a mold, rotating said mold at a speed sufficient to cause said abrasive grains to be concentrated in a radially outer region of said mold under the influence of centrifugal force with resin filling the interstices therebetween, causing the viscosity of said resin to increase during further continued rotation of said mold to a degree sulficient to prevent deleterious slumping of the grains when the speed of rotation is thereafter substantially reduced and the centrifugal efiect consequently likewise substantially reduced, substantially reducing said speed of rotation, foaming said now viscous resin in the interstices between said grains only sufficiently to space the latter very slightly apart and to assist in supporting said grains against slumping, and then setting said resin.

References Cited UNITED STATES PATENTS 3,052,927 9/1962 Hoppe et a1. 264311 XR 3,252,775 5/1966 Tocci-Guilbert 51--298 XR FOREIGN PATENTS 716,422 10/ 1954 Great Britain.

JAMES A. SEIDLECK, Primary Examiner.

P. E. ANDERSON, Assistant Examiner. 

